As a wrapping film for food, a film of a thermoplastic resin is generally used. The wrapping film needs physical strength as well as gas-barrier performance such as oxygen barrier performance from the viewpoint of keeping freshness of foods. In addition, the wrapping film needs moisture resistance in order to avoid the appearance of the wrapped article from getting worse even when preserving the wrapped article for a long term under a high humidity condition such as rainy season. Furthermore, since the wrapped article may be subjected to hot water treatment such as retort treatment, the wrapping film is required for avoiding the deterioration of film strength and gas-barrier performance as well as impaired appearance of the wrapped article by the hot water treatment.
It is difficult for a single film of one kind of thermoplastic resin to satisfy these requirements, and therefor a multilayer film in which various types of resin films are laminated is usually used for a wrapping film.
For example, used is a multilayer film where a film (gas barrier layer) having gas-barrier performance is sandwiched for protection by films (protection layers) each having strength and moisture resistance, or a multilayer film in which an adhesive resin layer is imposed between the gas barrier layer and the protection layer, for improving the binding strength between them.
A typical multilayer film exhibiting gas-barrier performance is a multilayer film where a saponified ethylene-vinyl ester-based copolymer (hereinafter, referred to as “EVOH resin”) used for gas barrier layer is sandwiched by polyolefin films.
However, there is known that the gas barrier performance of the wrapped article with this multilayer film employing EVOH resin for the gas-barrier layer is impaired by a hot water treatment such as retort treatment which exposes the wrapped article to hot water for long hours. The reason for the deterioration of the gas barrier performance of the multilayer film is supposed as follows: water comes into the EVOH resin layer from the outer edge of the multilayer film during the hot water treatment to destroy hydrogen bonds in EVOH resin, thereby allowing oxygen to come into the EVOH resin layer.
As a method of suppressing the deterioration of the gas barrier performance by the hot water treatment, adding a hydrate-forming metal salt as a drying agent to EVOH resin is known. A technique is also known that polyamide-based resin is added to EVOH resin to improve a resistance to hot water treatment. For instance, the patent document 1 discloses a multilayer structure employing EVOH resin layer for a gas-barrier layer, which is sandwiched with layers of thermoplastic resin containing a dehydrated disodium succinate hydrate for a drying agent, and that the multilayer structure maintained excellent gas-barrier performance after the hot water treatment.